Electron discharge device employing cavity resonator



Nav.. 21, 1950 l.. F. BROADWAY ELECTRON DISCHARGE DEVICE EMPLOYING CAVITY RESONATOR Filed Sept. 21, 1945 'Jnvemor LEUNHRD I'EJRUHDWHY (Ittorncg Patented Nov. Z1, 1950 UNITED STATES ATENT OFFICE Leonard Francis Broadway, Ickenham, England,l assigner to Electric and. Musical industries,

Ltd. ofEngland Application September 21, 1945, SerialNo; 617,855 In Great Britain November 27, 1942 Section 1,1 ub1ic Law 690, August s, 1946" Patent expires November27, 1962 8 Claims;

This; invention relates to hollow electrical resonators and to electron discharge devices embodfying suoli resonators.

Inel'ectrical apparatusit isv well known to employ hollow resonators for a variety` of' purposes. One importanti use of' hollow resonators is in electron discharge devices in which an electron beam is passed through the resonator, the resonator', the resonator serving to impart velocity modulation to the beam or serving to extract energy from a velocity modulated beam. The physical dimensions of the resonator determine the resonant frequency thereof and it will be appreciated, therefore, that in general, the higher the resonant frequency the smaller the dimensions ofthe resonator. It has been suggested to operate4 aresonator at a harmonic frequency of higher order than the fundamental or first harmonic frequency to whichthe'v resonator is tuned so as to allow the dimensions of the resonator to be increased compared with the dimensions of a resonator having' a fundamental' frequency equal to that of the desired harmonic frequency. However, even if a resonator is operated in such manner the dimensions' ofI the resonator may still be inconveniently small and this is particularly so when thev resonators are employed in electron discharge devices'where it is frequently necessary to providefor some adjustment of tuning; Tuning is frequently effected bythe insertion of one or moretuning plungers and in such cases the space available in the resonator.v for the insertion off the plunger or plungers may beso small that the range of tuning' adjustment possible: is very limitedv and, furthermore, if it is desired"V to apply to or extract energy from tlieresonatory by means of a coupling'l'oop inserted in the resonator, there is a tendency for ilashover to occur between the resonator and the coupling loop, particularly whenoperating at high powers, if the clearance between the wall ofthe resonator andthe coupling loop -is small.

TheV present inventionV is concerned' with the problem of;- increasing the si'zeof a resonator without the-change in size alteringr substantially the frequency of the resonator and without substantially changing its modeof' oscillation. It will be understood that various modes of oscillation can be set up in hollow resonators and for any particular purpose for which'the resonatoris to be used it is desirable that the mode of 'oscillation remains unchanged whatever the ultimate size of the-resonator may be.

When resonators are constructed to-operate at harmonic frequenciesl of' higher' order than the;

fundamental frequency, at least one voltagenode orV point of zero electric field.` is set up intermediate the extremitiesof the resonator and I have found in considering the problem with which the present invention is concernedthat whilst the dimensions of the resonator in planes at right angles to the electric el'd set up inthe resonator when the latter is excited, largely determine its resonant frequency, it is, however, possible to increase the depth of a portion of the resonator, the depth of the resonator being measured along the lines of the electric eld which extend directly between opposite walls of the resonator, without substantially changing the resonant frequency or the mode of oscillation of the resonator providing that the change indepth occurs inthe vicinity where, in operation, a voltage node is set up.

According therefore to one feature of. the present linvention. thereA is provided a hollow electrical resonator which is such that when excited a voltagenode can be setup intermediate the extremities off the resonator and wherein the resonator comprises portions between which there is an abrupt change in depth, the change in depth occurring in the vicinity of said voltage node.

By virtue of the present invention it is', therefore', possible to provide a resonator in which one portionA thereofV is increased in depth compared4 with another portion without the change in depth substantially altering the resonant frequency ofthe resonator or its mode of oscillation. Thez invention can be applied to a wide variety of resonators andi in particular to resonators of the formv employed inelectron discharge devices of thevelocity modulation type. In these resonators, particularly those used inV klystrons, portions of the sidesl ofthe resonator` are disposedl close togetlier to form a gap through which an electron beam is caused to pass, the gap providing. a lumpedcapacity. In such al. resonator the depth of the resonator in the vicinity of the lumped capacity does, to some extent,- determine the-resonant` frequency ofthe resonator and hence for any particular frequency theA dimensions' of that portion of the resonator are xed. The depth, however, of that portion ofthe resonator beyond the lumped capacity can be increased providing the change in depth occurs in the vicinity of a voltage node. This portion of increased depth can therefore readily accommodate a tuning plunger or plungers and/ora coupling loop withouty the disadvantages referred to above.

According, therefore, to another feature of the invention there is provided a hollow electrical resonator of which portions oftheopposite-sides of the resonator are disposed in proximity to one another to afford a lumped capacity and the resonator is such that when excited a voltage node can be set up at a position remote from the lumped capacity and wherein the depth of that portion of the resonator beyond said position is abruptly increased, the change in depth occurring in the vicinity of said voltage node.

According to another feature of the invention, an electron discharge device is provided employing a hollow resonator which is such that when excited a voltage node can be set up intermediate the extremities thereof and in which opposite sides of the resonator are formed to provide a gap through which the electron beam of the device can be caused to pass in the direction of the depth of the resonator and wherein that portion of the resonator remote from the gap is abruptly increased in depth, the increase in depth occurring in the vicinity where said voltage node is set up.

In order that the said invention may be clearly understood and readily carried into effect it will now be more fully described with reference to the accompanying drawings in which:

Figure 1 is a sectional view through a resonator illustrating one mode of operation thereof,

Figure 2 is a longitudinal section of an electron discharge device embodying a resonator according to the invention,

Figures 3 and 4 are cross-sectional views of electron discharge devices employing different forms of resonators according to the invention, and

Figure 5 is a cross-sectional view through a different form of resonator according to the invention.

Referring first to Figure 1 of the drawing, the reference numeral I indicates in cross-section a hollow rectangular resonator. Such a resonator can be excited in a fundamental mode so as to set up electric field components within the resonator extending in the same direction. Furthermore, if the resonator is excited at a harmonic frequency of higher order than the fundamental frequency determined by the dimensions of the resonator, at least one voltage node will be set up intermediate the extremities of the resonator, a single voltage node being set up when the resonator is operated at the second harmonic, i. e., at twice the fundamental frequency and occurring, for example, in the plane indicated by the dotted lines 2-2 in Figure l. I have found that whilst the dimensions of the resonator shown in Figure 1 in planes at right angles to the electric field largely determine the resonant frequency of the resonator, it is possible to increase the depth of the resonator without changing its resonant frequency or its mode of oscillation providing the change in depth occurs in the vicinity where the voltage node is set up. The depth of the resonator is the dimension measured along the lines of the electric field which extend directly between opposite walls of the resonator. Thus, according to the invention it is possible to increase abruptly the depth of the resonator shown in Figure 1 in the manner indicated by the dotted lines without substantially changing the resonant frequency of the resonator and Without substantially changing its mode of oscillation providing the increase in depth occurs in the vicinity of the voltage node, i. e., in the plane 2 2. The enlarged portion of the resonatur can be considered as a resonator oscillating at its fundamental frequency since as there is a voltage node in the region where the change in depth occurs, obviously the nodal surface can be replaced by a metal conductor without disturbing the field of distribution appropriate to the mode of oscillation under consideration, i. e., the mode in which the electric eld components are as indicated by the arrows in Figure 1. Theoretically, in this mode of oscillation the frequency of the resonator is dependent only on its dimension in planes transverse to the electric eld components as stated above and not on the depth of the resonator although it is possible that if the depth of the resonator exceeds onehalf of the wavelength in air of the oscillations set up, other modes of oscillation may occur with nodal planes perpendicular to the arrows shown in' Figure 1. In practice, however, such modes have not been detected.

Figure 2 of the drawings illustrates a resonator constructed according to the invention as applied to an electron discharge device of the velocity modulation type. In this figure the reference numeral 3 indicates generally a hollow resonator which is of toroidal form and the opposite sides of which are provided with a pair of centrally disposed apertures 4 and 5 forming a gap through which an electron beam can be caused to pass. The electron beam emanates from a cathode 6 surrounded by a focussing shield 1. Electrons, in passing through the gap in the resonator, become velocity modulated and such velocity modulated stream may be utilized in any of the manners known in the art. In the example shown in the figure the electron beam is directed towards a reflecting electrode 8 maintained at a suitable potential to cause the electron beam to be reflected back through the gap in known manner so that the device shown can function to generate self-maintained oscillations. The electron beam in passing through the resonator causes the latter to be excited, setting up electric field components which extend across the gap substantially parallel to the axis of the resonator shown in Figure 2. The depth of the resonator shown in Figure 2 is, therefore, its dimension parallel to the axis thereof. With the form of resonator shown in Figure 2 it is possible by suitable phasing of the electron beam to cause the resonator to operate at a harmonic of higher order than its fundamental frequency, with the result that a voltage node or nodes can be set up intermediate the center of the gap and the periphery of the resonator. It is obvious, however, that only the odd-numbered higher harmonies, namely the third, fth, seventh, etc., in which a voltage loop or point of maximum electric el'd occurs at the central gap formed by apertures 4 and 5, can be used with a centrallylocated electron beam. In accordance with the invention, in order to increase the size of the resonator the latter comprises in effect two portions,9 and I0, the portion I0 of the resonator being abruptly increased in depth compared with the portion 9, the change in depth occurring in the vicinity where a voltage node is set up. In the resonator shown in Figure 2 the provision of the gap gives rise to a lumped capacity and the effect of this lumped capacity results in the dimensions of the portion 9 of the resonator being fixed both in its radial dimensions and in its depth for any particular frequency. Thus the portion 9 of the resonator cannot be increased in depth without changing the resonant frequency. However, according to the invention it is possible to :provide the .portion II uotincreased `depth ywithout :substantially changing the :resonant vfrequency or .the mode of oscillation of `:the resonator,thusz:aording a portion ofincreased vdimensions'whioh can more easily-'accommodate tuning .plungers Aor coupling loops.

In fthe resonatorzshown .in Figure'2, .it is '1arranged ith-at the central portion 9 of.the:1esona tor lies within an evacuated enevelope II, whilst the portion II! of tthe resonator lies outside the evacuated envelope. In order :to Vreduce losses toagminimum due to :the presence ofthe envelope the wall of thelatter where fit lies :between Vthe sides of `the resonator :is disposed at fa `:position corresponding .to :a position where, Ain operation, 'a voltage Vnode is set up. In the example shown, the envelope I I is disposed ,at .a position substantiallyfcoinciding with the-position vwhere the 'increase of depth of the .resonator occurs. llt will,

however, be appreciated that although .in the example #shown the `wall of the yenvelope vcoin- ;cides substantially with such position, yit is possible, providing that the resonator is operated at -a suitable harmonic frequency 4atfwhich 4more than one voltage node is set up, for :the wall of i voltage node from the apertures s andf, and ;vl

the .envelope II .is :located as close to the second node as possible. However, it may be preferred to increase the .depth 'of the `resonator at a position corresponding to Vthe iii-st Voltage node from the apertures 4 and Sand .to vdispose the envelope at the second voltage node.

Whilstthe invention has been described above as applied to two constructions of resonators, it will beunderstood that the invention is not lilimited thereto-,since itcan be applied to other types of resonators. ForexamplaFg-ure 3 ofthe-drawings illustrates the invention as applied 'to .an electron 'discharge device having A`an envelope .I'I

.and containing .a .resonator 3 having an .annular gap I2 through which a beam of electrons is arranged to flow radially. rlhe resonator shown in Figure 3 is of toroidal form generated by revolving the resonator section shown in Figure 2 about an axis at right angles to a line passing through the gap and having portions 3 and 9. A central annular cathode I3 is employed and the reflecting electrode comprises a ring I4 surrounding the annular gap.

Figure 4 shows the invention as applied to a resonator for use with an annular beam of electrons. In this case the resonator is also of toroidal form and is generated by revolving the resonator section shown in Figure 2 about an axis parallel to a line passing through the gap and having portions 9 and IIIf. The gap I5 in this resonator is also of annular form, whilst the cathode IB is ring-shaped for generating an annular beam of electrons and the reilecing electrode I'I is also of ring shape. The electrodes are all mounted within envelope I I',

Figure 5 of the drawings illustrates a further form of resonator having portions 9' and ID" to which the invention can be applied. This resonator is of to-roidal form but instead of the sides of the inner resonator portion 9 being parallel In the fifth .harf :they .are divergent from .the `gap at the 4.center 4-oisaid inner portionitothe periphery thereof. kil resonator of this form has a depth which gradually increases from the -center to the periphery of .theinner portion and in accordance with the invention Vat the point where a voltage node Yis set up anabrupt change of de-pth occurs. The wordabrupt=" yreferred to herein is used in `order to .distinguish from a gradual kcha-nge 5in depth which occurs withthe inner resonator portion of the form shown Vin Figure 5. Theinvention can alsofbe applied toresonators for --use with ribbonshaped beams of electrons. The cross-section of a resonator for use with such a beam may be the same aslthat shown v'in Figure '2, the apertures in the resonator, however, being of elongated form and thegeneral shape `of the resonator conforming to the shape ofthe apertures. The vinvention is, of course,not limited in its application to the `particular forms of discharge devices shown 'in Figures `2, 3 and 4.

While I have indicated the preferred embodiments of my invention of'which I am now aware and have valso indicated only one specific application for which 'my invention may be employed, vit will be apparent Athat my invention is by no means limited to 'the exact forms villustrated or the use indicated, but that many variations'may be made inthe particular structure used andthe 0izrurpose 'for which it is employed without departing 'from the scope of my invention `as set 'forth in the appended claims.

Vlihat'l claim as new is:

1. A cavity1 ,resonator having a predetermined Afundamental frequency, including an enclosed hollow body formed with apertures in the opposite walls and a gap adjacent said apertures, .the .distance'between theinner Walls of saidbody adria-cent .said .apertures being substantially less than the distancebetween said walls at portions .remotefromsaid apertures, the Adistance between the walls beingr increased abruptly at a point vintermediate said apertures and Athe most remote portions of theresonatorin the vicinity of a voltage node when excited at an odd harmonic of higher order than said fundamental frequency.

'2. A cavity resonator 'having a predetermined 'fundamental frequency including a hollow conducting "body'having a central aperture 'therethrough, said body comprising iirst and second portions of substantially different depths joined v together, the depth of said body being abruptly increased at the junction of said portions, the change in depth being located at a point intermediate said central aperture and the outer periphery of said body in the vicinity of a voltage node when said resonator is excited at the fth harmonic of said fundamental frequency.

3. A cavity resonator having a predetermined fundamental frequency including a hollow conducting body comprising rst and second portions of substantially diierent depths joined together, the depth of said body being abruptly inlcreased at the junction of said portions, ther change in depth being located in the vicinity of `a voltage node when said resonator is excited at a harmonic of higher order than said fundamental frequency, the opposite walls of said portion of lesser depth having curved parts spaced from each other to define a gap.

4. A cavity resonator having a predetermined fundamentalfrequency comprising a hollow con-- ducting body formed by a surface of revolution 75 and having apertures in oppositely disposed sides thereof, the transverse section of said resonator outwardly from said apertures comprising a rst portion of an annulus having spaced curved reentrant parts defining said apertures and a second portion of an annulus remote from said apertures and of greater depth than and joined at an extremity of said first portion of an annulus, there being an abrupt increase in the depth of said resonator body at the junction of said portions, the change in depth being located in the vicinity of a voltage node when said resonator is excited at a harmonic of higher order than said fundamental frequency.

5. A cavity resonator as claimed in claim 4 in combination with an electron discharge device having a cathode for supplying a beam of electrons, and an electrode in the path of said electrons, said cavity resonator being positioned between and having its apertures aligned with said cathode and said electrode, whereby said resonator may be excited at a frequency considerably higher than a resonator of uniform depth equal to the depth of said second resonator portion.

6. An electron discharge device having a cathode for supplying a beam of electrons, an electrode lying in the path of said electrons, `and a cavity resonator having a predetermined fundamental frequency positioned between said cathode and said electrode and including a hollow conducting body, said hollow conducting body having curved reentrant portions centrally thereof, the inner ends of said reentrant portions being spaced from each other to form a gapand being provided with apertures adjacent said gap and through which the beam path lies, the spacing between the Walls of said resonator at said reentrant portions being substantially less than the spacing between the walls remote from said portions, the change in dimensions increasing abruptly at a point intermediate said gap and the most remote part of the resonator body in the vicinity of a voltage node when the resonator is, excited at an odd harmonic of higher order than said fundamental frequency.

7. An electron discharge device having an annular cathode for supplying electrons in an annular beam and an annular electrode in the path of said electrons, a cavity resonator adapted to be excited at a harmonic of its fundamental frequency and positioned between said cathode and said electrode and including a hollow toroidal resonator, the apertured portion of the resonator next adjacent the electron path being formed with aligned apertures through which said path extends and a gap adjacent said apertures, the distance between the opposite walls of said resonator adjacent said gap being substantially less than the distance between the walls of said resonator at those portions remote from said gap, the distance between the walls changing abruptly at a point intermediate the gap and the portion of the resonator farthest removed from said gap, said change in dimension occurring in the vicinity of a voltage node when said resonator is excited at an odd harmonic of higher order than said fundamental frequency.

8. A resonator having a predetermined fundamental frequency lcomprising a hollow conducting body formed by a surface of revolution and having apertures in oppositely disposed sides thereof, the transverse section of said resonator outwardly from said apertures comprising two diverging lines adjacent said apertures and a portion of an annulus remote from said apertures and joined at an extremity of said lines, there being an abrupt increase in the dimensions of said cross section in the direction extending from said apertures to the portion of said resonator remote from said apertures at the juncture of said lines and said portion of said annulus, said abrupt increase being located in the vicinity of a voltage node when said resonator is excited at an odd harmonic of higher order than said fundamental frequency.

LEONARD FRANCIS BROADWAY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,259 690 Hansen et al Oct. 21, 1941 2,278,210 Morton Mar. 31, 1942 2,343,487 Steudel Mar. 7, 1944 2,407,667 Kircher l Sept. 17, 1946 2,409,179 Anderson Oct. l5, 1946 2,409,222 Morton Oct. 15, 1946 

